Patent Document 1 mentioned below discloses a projection bolt, which is of a configuration as shown in FIG. 5A. A projection bolt 20 is formed of iron, and includes a shaft part 21 having a male screw, a circular enlarged diameter part 22 formed integrally with the shaft part 21 and having a diameter larger than that of the shaft part 21, and a circular fusion bonding projection 23 arranged at the center of the enlarged diameter part 22 on the side opposite to the shaft part 21. The fusion bonding projection 23 is a circular bulged portion having a diameter slightly smaller than that of the enlarged diameter part 22, and is equipped with a tapered portion 24 of a small inclination angle and a crest 25 whose central portion is pointed.
Patent Document 2 mentioned below discloses a projection bolt, which is of a configuration as shown in FIG. 5B. The projection bolt 20 is formed of iron, and differs from that shown in FIG. 5A in the configuration of the fusion bonding projection. That is, the end surface of the enlarged diameter part 22 is formed as a tapered surface 26 gradually reduced in height toward the outer periphery, with a fusion bonding projection 27 being formed at the center thereof. The fusion bonding projection 27 has at the end surface thereof a tapered portion 28 of a small inclination angle and a crest 29 whose central portion is pointed.
When the projection bolt 20 of FIG. 5A, 5B is welded to a steel sheet part 30 by electric resistance welding, a fusion bonding state is attained in which a gap L1 is formed in the periphery as shown in FIG. 5C, or an overall fusion bonding state is attained in which there is no gap Ll as shown in FIG. 5D. In the drawings, the region filled with black is the fusion bonding portion, which is indicated by reference numeral 31.    [Patent Document 1 ] JP 07-223078 A    [Patent Document 2 ] JP 2004-174599 A